Many ground vehicle (e.g. automotive) radar systems in use today are only able to determine distance and a horizontal or azimuth angle to a target or object. The transmit-antenna and receive antenna for such a system are typically vertical arrays of radiator and detector elements or patches, respectively. However, it has been recognized that it is desirable to determine a vertical or elevation angle to the object so elevated objects such as a bridge or building overhang is not inadvertently designated as object that is in the travel path of the vehicle.
As automotive radar systems are especially sensitive to cost factors, the number of transmit-antenna outputs and receive-antenna inputs is generally held to a minimum. The number of transmit-antennas and receive-antennas influences the radar spatial capability in azimuth and elevation and also drives system cost. That is, more antennas provide better capability at the expense of increased cost. Prior attempts to add elevation resolution undesirably increased number of transmit and/or receive antennas with the attendant increase in system cost. In order to increase antenna gain, the receive-antenna may have multiple strings or arrays of detector elements. Parallel arrays increase spacing between the phase-centers of the sub-arrays which leads to grating lobes that cause an undesirably large variation in receive antenna sensitivity for various azimuth angles. One way to reduce the effects of grating lobes is to provide an analog beam-former designed to overlap the sub-arrays to effectively reduce the spacing between the phase-centers of the sub-arrays. However, this approach typically requires a complex multi-layer feed structure leading to undesirable higher cost.